Digital motion picture decoding apparatus and digital motion picture decoding method

ABSTRACT

A digital motion picture decoding apparatus comprising an input buffer memory for storing coded data to be decoded, a reproduced picture memory for storing the decoded picture data to be displayed, and a display picture deciding means for deciding a reproduced picture to be output from the picture data stored in the reproduced picture memory, further comprises: a program changing means for changing the type of the coded data to be decoded; a program change detecting means for detecting from the output of the program changing means that the type of the coded data to be decoded is changed; and a display state maintaining means for controlling the picture data output from the reproduced picture memory so as to maintain the display state of the reproduced picture which is currently displayed according to the decision of the display picture deciding means, when it is detected from the output of the program change detecting means that the change of the type of the coded data to be decoded makes the coded data not continuous in time sequence.

FIELD OF THE INVENTION

[0001] The invention relates to a digital motion picture decodingapparatus and a digital motion picture decoding method for decodingpicture data coded by variable length coding for digital storage media,broadcasting, and communication. In particular, it relates to a digitalmotion picture decoding apparatus and a digital motion picture decodingmethod capable of correctly decoding and reproducing coded data which ispartially discontinuous due to change of program during thereproduction.

BACKGROUND OF THE INVENTION

[0002] Nowadays, demands for general method for coding motion pictureand its attached sound are increasing for various applications includingdigital storage media, or television broadcasting and communication.Motion Picture Experts Group (MPEG) in the InternationalElectrotechnical Commission (IEC) of International StandardizationOrganization (ISO) has proceeded international standardization of motionpicture coding method and already standardized several methods includingISO/IEC 13818-2 (MPEG2video).

[0003] Here a description is given, with reference to FIG. 9, of aconventional digital motion picture decoding apparatus for decoding thedata coded by the MPEG coding method which is disclosed by JapanesePublished Patent Application Number H8-32927, for example.

[0004] Reference numeral 900 designates a conventional digital motionpicture decoding apparatus, numeral 911 designates a channel selectingmeans which takes coded video streams corresponding to plural programsas inputs and, in response to a program select signal input by a user,selects a bit stream corresponding to one of the plural programs tooutput the bit stream as coded picture data 901. This means is alsoconfigured to output a program change signal at the same time. Theoutput coded picture data 901 passes through a code pattern detectingmeans 903 which is described later and is sequentially stored in aninput buffer memory 902.

[0005] The code pattern detecting means 903 is a block which inhibitsdata from passing through to the input buffer memory 902 until aprescribed code pattern, I picture (intra-coded picture) of GOP (Groupof pictures), is detected based on a change detection signal from thecoded data change detecting means 904, and this means normally allowsthe data to pass through during the reproduction processing.

[0006] Reference numeral 905 designates a decoding means which makes arequest for data to the input buffer memory 902, where the data obtainedis subjected to decoding processing and then successively stored into apredetermined reproduced picture area in a reproduced picture memory907.

[0007] A display picture deciding means 906 decides a reproduced pictureto be displayed according to time information of each reproduced pictureand the information about the data stored in the reproduced picturememory 907 obtained from a time information managing means 908 and areproduced picture area managing means 909, outputting the picture as areproduced picture data 910 for reproduction and display.

[0008] Next, a description is given of the operation for a transitionfrom normal reproduction mode to special reproduction mode in whichreproduction speed is changed to realize high-speed reproduction.

[0009] In response to a request for high-speed reproduction from a user,the coded picture data 901 is transmitted to the code pattern detectingmeans 903 in a format which comprises only an intra-frame coded picture(I picture) extracted from the bit stream recorded in record medium, andby detecting a pattern of contiguous I pictures, it is detected that thetransition to special reproduction mode has been completed. Followingthis, sending data through to the input buffer memory 902 is stopped anda clear signal is output from the code pattern detecting means 903 tothe input buffer memory 902, whereby the data stored in the input buffermemory 902 is erased.

[0010] When the code pattern detecting means 903 detects normal patternagain, cancel of the special reproduction mode is recognized and ablue-back processing which is described later is canceled, wherebysending data to the input buffer memory 902 is resumed.

[0011] Next, a description is given of the operation in theabove-described configuration when the coded data in the process ofreproduction becomes partially discontinuous due to change of program.

[0012] After detecting based on a program change detection signal thatchange of the coded data to be input has been performed, the coded datachange detecting means 904 sends a clear signal to the input buffermemory 902 to clear all the data stored in the memory 902, while sendinga data passing inhibit signal to the code pattern detecting means 903 inorder to inhibit data from passing through to the input buffer memory902 until a start code (I picture) of an intra-frame coded picture isdetected by the code pattern detecting means 903.

[0013] At this time, blank pictures (Bu pictures) are inserted followinga bidirectional predictive coded picture B3 (B picture), resulting inblue-back processing which provides a screen turned to blue.

[0014] Then, when the code pattern detecting means 903 detects the startcode (I picture) of the intra-frame coded picture from the coded picturedata after the change, passing data through to the input buffer memory902 is permitted again and the blue-back processing is canceled, wherebyreproduction operation is started again.

[0015] The decoding apparatus is configured to prevent reference pictureerror caused by connecting data arrays of different types at the changeof coded data and avoid decoding the data left in the buffer at thechange of the coded picture data, by performing the above-describedprocessing.

[0016] The prior art digital motion picture decoding apparatus and thedigital motion picture decoding method have the above-describedconstruction which prevents reference picture error caused by connectingdifferent types of data arrays at the change of coded data, but both theconventional decoding apparatus and the method have a problem that whenthe coded data is changed due to the change of program or other causes,blue-back screen is maintained during a prescribed period of time,making a viewer feel unconformable.

[0017] Moreover, in the case that change of coded data is not performedon a single piece of picture data basis and the coded data to be inputis changed in the process of decoding a single piece of picture data bymeans of the decoding means 905, the operation of changing program inthe middle of decoding processing as shown in FIG. 8(a) allows asituation as shown in FIG. 8(b) which is equal to the situation causedby inputting a data array to which a different type of data array isconnected, resulting in decoding error and the considerably confuseddisplay resulted from error picture.

[0018] Furthermore, as shown in FIG. 5, since the structure of the datacoded by a method employing bidirectional coding such as MPEG coding hasan order different from both the order in which the picture data of thecoded data is placed and the display order for reproducing pictures,forward predictive coded picture (P pictures), which is to be displayedfollowing a sequence of bidirectional predictive coded pictures (Bpictures), is decoded before decoding of the bidirectional predictivecoded pictures (B pictures), and the resultant decoded data is stored inthe reproduced picture memory 907 Therefore, as shown in FIG. 6), in theconfiguration only capable of clearing the data in the input buffermemory at the last stage, when the decoding processing has been alreadycompleted at the change of the coded picture data to be input andreproduced picture data which is not displayed yet (P6) is stored in thereproduced picture memory because the display time is not reached, thatreproduced picture data is certainly displayed when the display timebased on time information of the reproduced picture is reached.Therefore, with a long interval between each display time of reproducedpictures, an extended period of time is required before displaying thereproduced picture data which is unnecessary (P6) and it takes much timeto change the coded picture data in the process of reproduction, wherebythe blue-back screen is displayed for an extended period of time andmakes a viewer feel uncomfortable.

SUMMERY OF THE INVENTION

[0019] An object of the invention is to provide a digital motion picturedecoding apparatus and a digital motion picture decoding method which donot display blue-back screen during change of coded data due to changeof program or the like, in order to prevent a viewer from feelinguncomfortable.

[0020] A further object of the invention is to provide a digital motionpicture decoding apparatus and a digital motion picture decoding methodwhich do not cause decoding error even when coded data is not changed ona single piece of picture data basis and the coded data to be input ischanged when a single piece of picture data is being subjected todecoding processing.

[0021] A still further object of the invention is to provide a digitalmotion picture decoding apparatus and a digital motion picture decodingmethod which require a short time before displaying the reproducedpicture data which is unnecessary and do not take much time to changethe coded picture data in the process of reproduction, in the case thatone piece of picture data, whose display time established according totime information on a display picture basis is not reached, has beenalready decoded at the change of coded picture data, and the reproducedpicture is stored in the reproduced picture memory.

[0022] Other objects and advantages of the invention will becomeapparent from the detailed description that follows. The detaileddescription and specific embodiments described are provided only forillustration since various additions and modifications within the scopeof the invention will be apparent to those of skill in the art from thedetailed description.

[0023] According to a first aspect of the present invention, a digitalmotion picture decoding apparatus comprising an input buffer memory forstoring coded data to be decoded, a reproduced picture memory forstoring the decoded picture data to be displayed, and a display picturedeciding means for deciding from the picture data stored in thereproduced picture memory a reproduced picture to be output, furthercomprises: a program changing means for changing the type of the codeddata to be decoded; a program change detecting means for detecting fromthe output of the program changing means that the type of the coded datato be decoded is changed; and a display state maintaining means forcontrolling the picture data output from the reproduced picture memoryso as to maintain the display state of the reproduced picture beingcurrently displayed according to the decision of the display picturedeciding means, when it is detected from the output of the programchange detecting means that the change of the type of the coded data tobe decoded makes the coded data not continuous in time sequence.

[0024] According to a second aspect of the present invention, in thedigital motion picture decoding apparatus defined in the first aspect,the program change detecting means detects from an output of the programchanging means a difference between a normal reproduction state and areproduction state after change of program or a reproduction state at areproduction rate different from the normal reproduction rate.

[0025] According to a third aspect of the present invention, the digitalmotion picture decoding apparatus defined in the first aspect furthercomprises an input buffer memory erasing means for erasing the codeddata to be decoded which is stored therein, when the program changedetecting means detects that the data subjected to decoding is notcontinuous in time sequence.

[0026] According to a fourth aspect of the present invention, a digitalmotion picture decoding apparatus comprising an input buffer memory forstoring coded data to be decoded, a reproduced picture memory forstoring decoded picture data to be displayed, and a display picturedeciding means for deciding from the picture data stored in thereproduced picture memory a reproduced picture to be output, furthercomprises: a program changing means for changing the type of the codeddata to be decoded; a program change detecting means for detecting fromthe output of the program changing means that the type of the coded datato be decoded is changed; a reproduced picture memory nullifying meansfor nullifying the data stored in the reproduced picture memory otherthan the data corresponding to the reproduced picture being currentlydisplayed, when it is detected from the output of the program changedetecting means that the change of the type of the coded data to bedecoded makes the coded data not continuous in time sequence.

[0027] According to a fifth aspect of the present invention, the digitalmotion picture decoding apparatus defined in the fourth aspect comprisesa reproduced picture area managing means for managing memory areainformation used when the decoding means writes the decoded picture datainto the reproduced picture memory, and in this apparatus, thereproduced picture memory nullifying means erases the memory areainformation corresponding to the data stored in the reproduced picturearea managing means except the data corresponding to a reproducedpicture being currently displayed.

[0028] According to a sixth aspect of the present invention, in thedigital motion picture decoding apparatus defined in the fourth aspect,the reproduced picture memory nullifying means erases the data in thereproduced picture memory except the data corresponding to thereproduced picture being currently displayed.

[0029] According to a seventh aspect of the present invention, in thedigital motion picture decoding apparatus defined in the fourth aspect,picture data to be decoded next is written into a nullified memory areaof the reproduced picture memory.

[0030] According to an eighth aspect of the present invention, a digitalmotion picture decoding apparatus comprising an input buffer memory forstoring coded data to be decoded, a reproduced picture memory forstoring decoded picture data to be displayed, and a display picturedeciding means for deciding from the picture data stored in thereproduced picture memory a display picture to be output, furthercomprises: a program changing means for changing the type of the codeddata to be decoded; a program change detecting means for detecting fromthe output of the program changing means that the type of the coded datato be decoded is changed; and a decoding stopping means for stoppingdecoding processing by the decoding means and controlling the decodingmeans such that the data being in the process of decoding at that timeis written into the reproduced picture memory as the already decodedone, when it is detected from the output of the program change detectingmeans that the change of the type of the coded data to be decoded makesthe coded data not continuous in time sequence.

[0031] According to a ninth aspect of the present invention, a digitalmotion picture decoding method comprises the steps of: detecting whetherdata is continuous in time sequence during decoding processing; andmaintaining the display state of a reproduced picture being currentlydisplayed when it is found in the first step that the data is notcontinuous in time sequence during the decoding processing.

[0032] According to a tenth aspect of the present invention, a digitalmotion picture decoding method comprises the steps of: detecting whetherdata is continuous in time sequence during decoding processing; andnullifying data other than the one corresponding to a reproduced picturebeing currently displayed, when it is judged in the first step that thedata is not continuous in time sequence during the decoding processing.

[0033] According to an eleventh aspect of the present invention, thedigital motion picture decoding method defined in the tenth aspectfurther comprises a step of stopping decoding processing and processingthe data being currently decoded at that time as the already decodedone, when it is judged in the first step that the data is not continuousin time sequence during the decoding processing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a block diagram of a digital motion picture decodingapparatus according to an embodiment of the present invention.

[0035]FIG. 2 is a flow chart for explaining the operation during changeof program by the digital motion picture decoding apparatus of theembodiment of the invention.

[0036]FIG. 3 is a flow chart for explaining the operation of thedecoding means during change of program in the digital motion picturedecoding apparatus of the embodiment of the invention.

[0037]FIG. 4 is a flow chart for explaining the operation for displayingnew coded data after change of program in the digital motion picturedecoding apparatus of the embodiment of the invention.

[0038]FIG. 5 is a diagram showing an example of the structure of a bitstream coded in a coding mode which uses a bidirectional predictivecoding such as MPEG.

[0039]FIG. 6 is a diagram showing an example of the structure of a bitstream and an exemplary operation of the conventional digital motionpicture decoding apparatus, in the case that coded data to be input ischanged in the process of reproduction.

[0040]FIG. 7 is a diagram illustrating an exemplary operation of adigital motion picture decoding apparatus of the present invention underthe condition illustrated in FIG. 5.

[0041] FIGS. 8(a) to 8(c) are diagrams showing the states of a decodingmeans in the case that coded data is changed in the middle of decodingone piece of picture data.

[0042]FIG. 9 is a block diagram of a prior art digital motion picturedecoding apparatus.

[0043]FIG. 10 is a conceptual diagram of a state table of reproducedpicture areas which constitute the digital motion picture decodingapparatus according to the embodiment of the present invention.

PREFERRED EMBODIMENT OF THE INVENTION

[0044] Hereinafter a description is given of a digital motion picturedecoding apparatus and a digital motion picture decoding methodaccording to the present invention with reference to drawings.

[0045]FIG. 1 is a block diagram of a digital motion picture decodingapparatus according to an embodiment of the invention. In the figure,reference numeral 100 designates a digital motion picture decodingapparatus of the invention, and numeral 104 designates a program changedetecting means, which means takes coded video streams corresponding toa plurality of coded programs as inputs and, according to a programselect signal input by a user through a control means (remotecontroller) 111, selects one bit stream corresponding to one of theplural programs to output it as coded picture data 101.

[0046] In normal decoding operation, the input coded picture data 101 ispassed through a code pattern detecting means 103 which is describedlater, and stored in an input buffer memory 102.

[0047] The input buffer memory 102 has a relatively large capacity, forexample, about 400 Kb, and outputs the stored coded data according to arequest from a decoding means 105 which is also described later.

[0048] The decoding means 105 manages an amount of data in the inputbuffer memory 102 based on the amount of the data input into this meansand the amount of the data output from the means, and controls the datainput to the means itself according to the amount of the data stored inthe input buffer memory 102. Moreover, the decoding means 105 alsoincludes a reproduced picture memory address management table formanaging null addresses when the decoded picture data is written into areproduced picture memory 107 which is also described later.

[0049] The code pattern detecting means 103 is a block which inhibitsdata from passing through to the input buffer memory 102 until apredetermined code pattern is detected, according to a data passinginhibit signal from the program change detecting means 104, and thismeans normally allows data to passing through during reproduction.

[0050] The reproduced picture memory 107, which comprises SDRAM, forexample, has a capacity for storing three frames of picture data (500Kb×3=1.5 Mb) and includes divided regions M0 to M2. The coded picturedata input to the decoding means 105 to be decoded is stored into one ofthe reproduced picture regions M0 to M3 in the reproduced picture memory107, on one display picture basis.

[0051] The display picture deciding means 106 comprises a base timecounter for counting up with time precision equal or superior to thetime precision of the time information established on single framepicture basis by a time information managing means 108 which isdescribed later, and this base time counter manages a base time forreproduction operation. Moreover, the base time counter can change itscount according to the base time information signal input from thedecoding means 105. Then, when it is judged from a comparison betweenthe count of the base time counter and a value of the time informationfrom the reproduced picture area managing means 109 which is describedlater that these values are equal or approximately equal, this meansdecides a reproduced picture area as an output area according to thedata storage information from the reproduced picture area managing means109. Then, to output the decoded data in this area, the deciding meansoutputs a signal for selecting a reproduced picture area from whichdecoded data is to be output (this signal is hereinafter referred to asan output area-reproduced picture area select signal), to the reproducedpicture memory 107 and the reproduced picture area managing means 109.

[0052] Reference numeral 109 designates the reproduced picture areamanaging means, and this managing means relates the informationindicating which reproduced picture area of the reproduced picturememory 107 stores the decoded picture data, to a display state flag fedback from the display picture deciding means 106 which is describedlater, and performs management using a table as shown in FIG. 10, forexample.

[0053] Further in the above configuration, the control means (remotecontroller) 111 implements a program changing means, and the programchange detecting means 104 implements a program change detecting meansfor detecting change of program according to a program select signalinput from the control means (remote controller) 111, and an inputbuffer memory erasing means for erasing the data stored in the inputbuffer memory 102. The display picture deciding means 106 and thereproduced picture area managing means 109 implement a display statemaintaining means and the program change detecting means 104 and thereproduced picture area managing means 109 implement a reproducedpicture memory nullifying means and a decoding stopping means.

[0054]FIGS. 2 and 4 are flow charts illustrating the processes performedby the digital motion picture decoding apparatus, and the operation willbe described with reference to these flow charts.

[0055] Before the description, it is assumed that among the picture datastored in the reproduced picture memory 107, a piece of picture datastored in reproduced picture area M1 is currently reproducedcorresponding to a specified program selected by user input.

[0056] Next, a description is given of the operation performed in thecase that data array subjected to decoding processing becomesnon-continuous at some midpoint because coded data is changed during thereproduction, with reference to the block diagram of FIG. 1 and the flowchart of FIG. 2.

[0057] Program change processing in the middle of reproduction starts asfollows; in step S1, the control means (remote controller) 111 changes aprogram desired to be reproduced in the process of reproduction andoutputs to the program change detecting means 104 a program selectsignal for changing coded data to be reproduced.

[0058] Then, in step S2, the program change detecting means 104 outputsto the code pattern detecting means 103 a data passing inhibit signalfor inhibiting coded data from passing through until a prescribed codeis detected, and simultaneously outputs to the input buffer memory 102 adata clear signal for clearing all the data currently stored in thismemory 102. Further, at the same time, this means also outputs to thedecoding means 105 a decoding stop signal for stopping the currentlyperformed decoding, and to the reproduced picture area managing means109, a flag erase signal for clearing the data in the areas other thanthe area for a reproduced picture being currently displayed.

[0059] Thereafter, the program change detecting means 104 selects codeddata of a program, from the stream data arrays of plural programs whichhave been input, according to the program select signal from the controlmeans (remote controller) 111, and starts outputting the coded data tothe code pattern detecting means 103.

[0060] In response to the input of the data passing inhibit signal, thecode pattern detecting means 103 inhibits the input coded data frompassing through to the input buffer memory 102 and waits for the inputof a sequence header code of the coded data based on MPEG.

[0061] Further, the input buffer memory 102 clears all the stored datain response to the input data clear signal and waits the data input fromthe code pattern detecting means 103.

[0062] Furthermore, the decoding means 105 stops the currently performeddecoding processing in response to the input of the decoding stoppingsignal and waits for the input of a sequence header code. Thereafter,this means outputs a data request signal requesting new coded data tothe input buffer memory 102.

[0063] In step S3, when data is input, it is checked whether the data isa sequence header code or not. The data which is not the sequence headercode is discarded, and a check is made for next input data to seewhether the data is a sequence header code or not. The process in stepS3 is repeated until a sequence header code is input.

[0064] As described above, in the digital motion picture decodingapparatus of the conventional configuration, by stopping the decodingprocessing immediately after change of program, a state is brought aboutwhich is equal to the state that different types of data arraysconnected to each other are input to the decoding means. This results indecoding error, and considerably confused display caused by displayingerror pictures, which makes viewer feel uncomfortable.

[0065] Further, the reproduced picture area managing means 109 clearsthe data in the reproduced picture memory 107 except the data in thearea for the reproduced picture being currently used as a displayoutput, by means of a flag erase signal. More specifically, flags arechanged so that the data storage information about the reproducedpicture memory 107 indicates “no data” except the one about thereproduced picture area M1. This process resolves one of the problemsoccurred in the digital motion picture decoding apparatus of theconventional configuration that displaying unnecessary reproducedpicture data prolongs a time interval between change of the codedpicture data to be input and change of reproduced pictures.

[0066] Furthermore, data storage information and a time informationnotifying signal are output to the display picture deciding means 106,the information and the signal both being the contents of the reproducedpicture area state table changed to the state after data clearimmediately after clearing the data in the reproduced picture memory107.

[0067] According to the data storage information and the timeinformation notifying signal from the reproduced picture area managingmeans 109, the display picture deciding means 106 detects that there isno data in reproduced picture areas except the one being currently usedas an output area for display, and outputs to the reproduced picturememory 107 an output area-reproduced picture select signal whichinstructs to output the data from this reproduced picture arearepeatedly because the reproduced picture areas other than that areacontains no data This process resolves one of the problems for thedigital motion picture decoding apparatus of the conventionalconfiguration that blue-back display processing (see Bu in FIG. 6)performed until the display of a new picture after the change (see IO′in FIG. 6) causes a display state transition which is considerablyclumsy.

[0068] Next, a description is given of the operation in the process ofdisplaying new coded data obtained after change of program.

[0069] In step S3, the code pattern detecting means 103 analyzes newcoded data obtained from the program change detecting means 104 todetect a sequence header code and, when the sequence head code isdetected, the step is followed by step S4 in which passing coded datathrough to the input buffer memory 102 is permitted again.

[0070] This step is followed by step S5, in which storing coded datainto the input buffer memory 102 is resumed with the data following thesequence header code being initial one, and outputting the data isstarted corresponding to the data request signal from the decoding means105.

[0071] In step S6, from the coded data which includes display orderinformation for single frame picture being currently decoded and thetime information about the times at which the picture should bedisplayed, the decoding means 105 extracts the above information tooutput it as an extracted information signal to the time informationmanaging means 108. If the coded data contains no time information basedon which a single frame pictures being currently decoded is displayed,the display order information only is output to the time informationmanaging means 108 as an extracted information signal. Moreover, thedecoding means 105 extracts the base time information contained in thecoded data and outputs it as a base time information signal to thedisplay picture deciding means 106.

[0072] Following this, the same operation as in normal reproduction isperformed: as shown in the flow chart of FIG. 3, in step S7, thedecoding means 105 judges whether the input coded data is predictivecoded data or not; when the coded data is a predictive coded one, thisstep is followed by step S8 wherein a reference picture data requestsignal is output to the reproduced picture area managing means 109 torequest necessary data. If the input coded data is not a predictivecoded one, decoding processing is performed using the input coded dataonly.

[0073] Then, in step S9, the reproduced picture area managing means 109checks which reproduced picture area of the reproduced picture memory107 stores the requested reference picture data, and in step S10, thereproduced picture area managing means 109 outputs a reference picturedata output control signal which instructs to output the data in thespecified area as reference picture data.

[0074] In step S11, the reproduced picture memory 107 outputs the datain the specified area as reference picture data, according to theinstruction of the reference picture data output control signal.

[0075] Then, in step S12, the decoding means 105 starts decoding thepredictive coding type coded data using the input reference picturedata.

[0076] In step S12-1, the decoding means 105 outputs decoded dataobtained by the decoding processing to the reproduced picture memory 107The reproduced picture area managing means 109 outputs to the reproducedpicture memory 107 an address signal which instructs the area in whichthat decoded data is to be stored.

[0077] If the coded data in the process of decoding contains base timeinformation, the information is output to the display picture decidingmeans 106 as base time information.

[0078] Further, the display order information for displaying a singleframe picture being in the process of decoding and the time informationbased on which such picture is displayed are extracted from the codeddata and output as an extracted information signal to the timeinformation managing means 108. If the above-described time informationis not included in the coded data, the display order information only isextracted and output as an extracted information signal to the timeinformation managing means 108.

[0079] Next, the time information managing means 108 judges whether thetime information extracted from the coded data is included in anextracted information signal which is input, and when the timeinformation is included, this time information is determined as the timeinformation based on which the picture is to be displayed, which pictureis currently being decoded in the decoding means 105, and output as atime information signal to the reproduced picture area managing means109. If the time information is not found, time information based onwhich the single frame picture being currently decoded is to bedisplayed is calculated from time information and display orderinformation for another single frame picture, both previously input fromthe decoding means 105, and the display order information for the singleframe picture being currently decoded, outputting it as a timeinformation signal to the reproduced picture area managing means 109.

[0080] Next, in step S16, the reproduced picture area managing means 109checks the contents of the reproduced picture area state table,outputting new decoded data obtained from the decoding means 105 afterthe change of program to the reproduced picture areas other than thereproduced picture area from which the display is being output (ml),that is, M0 and M2, and from these areas the decoded data has beencleared. The reproduced picture area managing means 109 also changes thecontents of the reproduced picture area state table, outputting datastorage information and a time information notifying signal to thedisplay picture deciding means 106.

[0081] Then, in step S17, the display picture deciding means 106 detectsfrom the data storage information and the time information notifyingsignal the fact that new picture is ready to be output, outputting anoutput area-reproduced picture area select signal such that the decodeddata of the picture stored in one of the reproduced picture areas willbe output when the time based on the time information for the picture isreached.

[0082] Following this step, in step S18, the reproduced picture memory107 outputs new decoded data obtained after the change of program asreproduced picture data, according to the output area-reproduced picturearea select signal.

[0083] The above-described processes enable to change a picture to bedisplayed, according to the content of a newly selected program.

[0084] As described above, in the first embodiment, passing data throughto the input buffer memory 102 is inhibited when change of program isdetected, and simultaneously all the data in the input buffer memory 102is erased while the display picture deciding means 106 is controlled tomaintain the display picture at this time. This disables an unnaturalscreen such as a blue-back screen to be displayed for a certain periodof time during the change of program, reducing viewer'suncomfortableness.

[0085] Further, when the change of program is detected, the informationabout the areas in the reproduced picture memory 107 other than thatcurrently used as an output area for display is cleared to treat them asnull areas, whereby the display of the unnecessary picture is preventedwhich will be displayed after the change of coded data. Further, sincethe coded data obtained after the change is written into the reproducedpicture memory 107 more swiftly, it is possible to reduce a timenecessary to perform the process until the reproduced picture of thecoded data after the change is displayed.

[0086] In addition, since the decoding processing performed by thedecoding means 105 is stopped and a code indicating that decoding asingle piece of picture data is completed is added to the picture databeing currently decoded, the data array of a different program isconfused with the continuous data array being in the process ofdecoding, thereby preventing the occurrence of decoding error due tothis misconception and the resultant display of error pictures.

[0087] Although in the first embodiment the code pattern detecting means103 and the decoding means 105 have a feature of waiting for a sequenceheader code to be detected after the change of program, any detectionfeature is applicable as long as it can detect a code pattern indicatinga head of a data unit for decoding processing and this detecting targetis not limited to a sequence header code. Other code patterns such as anMPEG-based GOP header code and a start code of an intra-frame codedpicture (I picture) can be used as long as those codes are code patternswhich allow to reproduce picture only from the coded data following thedetected data only.

[0088] Although this embodiment is configured such that the code patterndetecting means 103 is placed as a stage followed by the input buffermemory, the same operation can be obtained by the followingconfiguration that the code pattern detecting means 103 is placed as astage following the input buffer memory 102, and the buffer memory 102outputs data according to the request for data from the code patterndetecting means 103 and inhibits the data to be sent to the subsequentdecoding means 105 until the detecting means 103 detects a predeterminedcode pattern.

[0089] Although in this embodiment it is assumed that the reproducedpicture memory 107 has three reproduced picture areas, the number of thereproduced picture areas is not limited to three and any number ofreproduced picture areas are applicable as long as the number allows thedecoded reproduced picture data to be displayed in a display order.

[0090] In addition, although in this embodiment the reproduced pictureareas in the reproduced picture memory 107 are illustrated as contiguousareas, the structure of the reproduced picture memory 107 is not limitedto this illustration and a reproduced picture memory can be employedwhich divides decoded data of each single picture data intonon-contiguous areas for storage.

[0091] Although in this embodiment a decoding object is MPEG-based codeddata, the coded data as a decoding object is not limited to this datacoded based on MPEG, and any coded data is applicable as long as thiscode is the one coded by bidirectional predictive coding and thereforethe data coded by any coding method other than MPEG-based coding may beused.

[0092] The stream data input to the program change detecting means 104is not necessary to be broadcasted and such configuration is applicablein which required program stream data is read from record media such asdisks.

[0093] Although in this embodiment the count of the base time counter ischanged according to base time information whenever that base timeinformation is input to the display picture deciding means 106, thischange may be performed only when the base time informationcorresponding to the initial part of each coded data to be reproduced isinput to the display picture deciding means 106, or this change may beperformed at arbitrary timing corresponding to the input of a controlsignal from external unit.

[0094] Furthermore, in this embodiment, it is not necessary for the basetime information to be changed according to the data extracted from thecoded data by the decoding means 105 and external unit may directly setthe count of the base time counter to an arbitrary value.

What is claimed is:
 1. A digital motion picture decoding apparatuscomprising an input buffer memory for storing coded data to be decoded,a reproduced picture memory for storing the decoded picture data to bedisplayed, and a display picture deciding means for deciding from thepicture data stored in the reproduced picture memory a reproducedpicture to be output, said decoding apparatus further comprising: aprogram changing means for changing the type of the coded data to bedecoded; a program change detecting means for detecting from the outputof the program changing means that the type of the coded data to bedecoded is changed; and a display state maintaining means forcontrolling the picture data output from the reproduced picture memoryso as to maintain the display state of the reproduced picture beingcurrently displayed according to the decision of the display picturedeciding means, when it is detected from the output of the programchange detecting means that the change of the type of the coded data tobe decoded makes the coded data not continuous in time sequence.
 2. Thedigital motion picture decoding apparatus defined in claim 1 wherein theprogram change detecting means detects from an output of the programchanging means a difference between a normal reproduction state and areproduction state after change of program or a reproduction state at areproduction rate different from the normal reproduction rate.
 3. Thedigital motion picture decoding apparatus defined in claim 1 furthercomprising an input buffer memory erasing means for erasing the codeddata to be decoded which is stored therein, when the program changedetecting means detects that the data subjected to decoding is notcontinuous in time sequence.
 4. A digital motion picture decodingapparatus comprising an input buffer memory for storing coded data to bedecoded, a reproduced picture memory for storing decoded picture data tobe displayed, and a display picture deciding means for deciding from thepicture data stored in the reproduced picture memory a reproducedpicture to be output, said decoding means further comprising: a programchanging means for changing the type of the coded data to be decoded; aprogram change detecting means for detecting from the output of theprogram changing means that the type of the coded data to be decoded ischanged; a reproduced picture memory nullifying means for nullifying thedata stored in the reproduced picture memory other than the datacorresponding to the reproduced picture being currently displayed, whenit is detected from the output of the program change detecting meansthat the change of the type of the coded data to be decoded makes thecoded data not continuous in time sequence.
 5. The digital motionpicture decoding apparatus defined in claim 4 comprising a reproducedpicture area managing means for managing memory area information usedwhen the decoding means writes the decoded picture data into thereproduced picture memory, wherein the reproduced picture memorynullifying means erases the memory area information corresponding to thedata stored in the reproduced picture area managing means except thedata corresponding to a reproduced picture being currently displayed. 6.The digital motion picture decoding apparatus defined in claim 4 whereinthe reproduced picture memory nullifying means erases the data in thereproduced picture memory except the data corresponding to thereproduced picture being currently displayed.
 7. The digital motionpicture decoding apparatus defined in claim 4 wherein picture data to bedecoded next is written into a nullified memory area of the reproducedpicture memory.
 8. A digital motion picture decoding apparatuscomprising an input buffer memory for storing coded data to be decoded,a reproduced picture memory for storing decoded picture data to bedisplayed, and a display picture deciding means for deciding from thepicture data stored in the reproduced picture memory a display pictureto be output, said decoding apparatus further comprising: a programchanging means for changing the type of the coded data to be decoded; aprogram change detecting means for detecting from the output of theprogram changing means that the type of the coded data to be decoded ischanged; and a decoding stopping means for stopping decoding processingby the decoding means and controlling the decoding means such that thedata being in the process of decoding at that time is written into thereproduced picture memory as the already decoded one, when it isdetected from the output of the program change detecting means that thechange of the type of the coded data to be decoded makes the coded datanot continuous in time sequence.
 9. A digital motion picture decodingmethod comprising the steps of: detecting whether data is continuous intime sequence during decoding processing; and maintaining the displaystate of a reproduced picture being currently displayed when it is foundin the first step that the data is not continuous in time sequenceduring the decoding processing.
 10. A digital motion picture decodingmethod comprising the steps of: detecting whether data is continuous intime sequence during decoding processing; and nullifying data other thanthe one corresponding to a reproduced picture being currently displayed,when it is judged in the first step that the data is not continuous intime sequence during the decoding processing.
 11. The digital motionpicture decoding method defined in claim 10 further comprising a step ofstopping decoding processing and processing the data being currentlydecoded at that time as the already decoded one, when it is judged inthe first step that the data is not continuous in time sequence duringthe decoding processing.